The present invention relates to a process, an apparatus and a slurry for wire sawing, for example, of crystalline material to form semiconductor wafers.
EP 0 837 115 B1 discloses a process for cutting off wafers from a crystal made from hard, brittle material using a wire saw and an agent in the form of a suspension. The agent substantially comprises a non-aqueous liquid, in which hard-material particles with an abrasive action are dispersed. The non-aqueous liquid is selected from a group of compounds consisting of polyglycols with a molecular weight of from 75 to 150 and mixtures thereof containing at most up to 5% by weight of water, based on the weight of the suspension. The viscosity of the liquid is preferably 50–800 mPas at 20° C. The process has the drawback that the sawing suspension which is used has to be replaced at relatively frequent intervals, and the properties of the slurry may change in an uncontrolled way during wire sawing.
CH 691 039 A5 discloses the use of a slurry which consists of a mixture of polyols, at least one of which is a polyglycol, in wire sawing. The mixture of polyols can be selected in such a way that the slurry has a predetermined viscosity. The document also states that the mixture of polyols may contain water.
EP 0 686 684 A1 discloses the use of a sawing suspension which consists of an abrasive in liquid aqueous phase. The sawing suspension contains a thickener, such as, for example, water-soluble polymers. The viscosity of the sawing suspension is between 10 and 1000 mPas at a shear rate of 10 s−1.
DE 199 38 339 A1 discloses a sawing suspension for use in a wire saw, which contains at least one member selected from the group consisting of mineral oils and glycols. At least one further additive selected from the group of the polysiloxanes is also used.
U.S. Pat. No. 6,422,067 describes how a slurry which is suitable for wire sawing has a viscosity of approximately 400–700 mPas at a shear rate of approximately 2 s−1 and of 50–300 mPas at a shear rate of approximately 380 s−1.
A wire sawing process which is known to the assignee of this application (not prior art; hereinafter referred to a “Comparison”) uses a glycol-based slurry which contains a glycol-based carrier liquid and SiC grains (SiC Fujimi GC 1000) as hard material with an abrasive action. The water content of the slurry remains constant during the wire sawing.
FIGS. 11 and 12 illustrate the changes in a glycol-based slurry according to the Comparison, comprising the carrier substance known by the product name Pluriol E 200, as the duration of use for the wire sawing of GaAs crystals increases. In this case, the water content of the slurry remains approximately constant at (12±2) g/l. The dynamic viscosity of the slurry at a shear rate of 20.4 s−1 is plotted against the GaAs content of the slurry in the upper part of FIG. 11. An increasing GaAs content corresponds to an increasing duration of use of the slurry for the wire sawing of GaAs. The density of the slurry is plotted against the GaAs content in the lower part of FIG. 11. The dynamic viscosity and the density of the slurry increase approximately linearly with the GaAs content. The dynamic viscosity of the slurry for various GaAs contents for the Comparison is plotted against the shear rate in FIG. 12. The slurry in this case has a non-Newtonian behavior for each of the GaAs contents shown.
FIG. 13 shows the properties of a slurry according to the Comparison which contains a glycol-based carrier substance known by the product name Betronol MF V 1016 and has an approximately constant water content of (18±5) g/l. The figure plots the dynamic viscosity of the slurry for various GaAs contents against the shear rate. As above, the dynamic viscosity increases with an increasing GaAs content, i.e., as the duration of use of the slurry increases. At the same time, the slurry has a non-Newtonian behavior for each GaAs content.
The Comparison process known to the assignee of this application has the drawback that the changes in the properties of the slurry with increasing duration of use simultaneously cause a deterioration in the quality of the GaAs wafers produced in this way. In particular, structures which reveal the orientation of the wire relative to the GaAs wafers during sawing are visible to the naked eye on the surface of the GaAs wafers after wire sawing. These structures mean that the back surface, even of wafers which have undergone single-side polishing, has to be re-machined, for example, by surface lapping, before they can be delivered to component manufacturers or prior to the epitaxial production of components.